14 files changed, 471 insertions, 2 deletions

diff --git a/src/mongo/db/commands/rwc_defaults_commands.cpp b/src/mongo/db/commands/rwc_defaults_commands.cpp
index f02cd506186..7fd405804f9 100644
--- a/src/mongo/db/commands/rwc_defaults_commands.cpp
+++ b/src/mongo/db/commands/rwc_defaults_commands.cpp
@@ -42,10 +42,14 @@
 #include "mongo/db/repl/replication_coordinator.h"
 #include "mongo/logv2/log.h"
 #include "mongo/rpc/get_status_from_command_result.h"
+#include "mongo/util/fail_point.h"
 
 namespace mongo {
 namespace {
 
+// Hang during the execution of SetDefaultRWConcernCommand.
+MONGO_FAIL_POINT_DEFINE(hangWhileSettingDefaultRWC);
+
 /**
  * Replaces the persisted default read/write concern document with a new one representing the given
  * defaults. Waits for the write concern on the given operation context to be satisfied before
@@ -113,6 +117,17 @@ public:
         auto typedRun(OperationContext* opCtx) {
             assertNotStandaloneOrShardServer(opCtx, SetDefaultRWConcern::kCommandName);
 
+            auto replCoord = repl::ReplicationCoordinator::get(opCtx);
+            auto wcChanges = replCoord->getWriteConcernTagChanges();
+
+            // Synchronize this change with potential changes to the write concern tags.
+            uassert(ErrorCodes::ConfigurationInProgress,
+                    "Replica set reconfig in progress. Please retry the command later.",
+                    wcChanges->reserveDefaultWriteConcernChange());
+            ON_BLOCK_EXIT([&]() { wcChanges->releaseDefaultWriteConcernChange(); });
+
+            hangWhileSettingDefaultRWC.pauseWhileSet();
+
             auto& rwcDefaults = ReadWriteConcernDefaults::get(opCtx->getServiceContext());
             auto newDefaults = rwcDefaults.generateNewCWRWCToBeSavedOnDisk(
                 opCtx, request().getDefaultReadConcern(), request().getDefaultWriteConcern());
@@ -120,8 +135,7 @@ public:
             // because it only has to exist on the actual shards in order to be valid.
             if (serverGlobalParams.clusterRole != ClusterRole::ConfigServer) {
                 if (auto optWC = newDefaults.getDefaultWriteConcern()) {
-                    uassertStatusOK(
-                        repl::ReplicationCoordinator::get(opCtx)->validateWriteConcern(*optWC));
+                    uassertStatusOK(replCoord->validateWriteConcern(*optWC));
                 }
             }
 
diff --git a/src/mongo/db/repl/repl_set_config.cpp b/src/mongo/db/repl/repl_set_config.cpp
index 86368756c7f..902435b53e0 100644
--- a/src/mongo/db/repl/repl_set_config.cpp
+++ b/src/mongo/db/repl/repl_set_config.cpp
@@ -766,6 +766,29 @@ ReplSetConfigPtr ReplSetConfig::getRecipientConfig() const {
     return _recipientConfig;
 }
 
+bool ReplSetConfig::areWriteConcernModesTheSame(ReplSetConfig* otherConfig) const {
+    auto modeNames = getWriteConcernNames();
+    auto otherModeNames = otherConfig->getWriteConcernNames();
+
+    if (modeNames.size() != otherModeNames.size()) {
+        return false;
+    }
+
+    for (auto it = modeNames.begin(); it != modeNames.end(); it++) {
+        auto swPatternA = findCustomWriteMode(*it);
+        auto swPatternB = otherConfig->findCustomWriteMode(*it);
+        if (!swPatternA.isOK() || !swPatternB.isOK()) {
+            return false;
+        }
+
+        if (swPatternA.getValue() != swPatternB.getValue()) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
 MemberConfig* MutableReplSetConfig::_findMemberByID(MemberId id) {
     for (auto it = getMembers().begin(); it != getMembers().end(); ++it) {
         if (it->getId() == id) {
diff --git a/src/mongo/db/repl/repl_set_config.h b/src/mongo/db/repl/repl_set_config.h
index 5e410b65963..847677cd292 100644
--- a/src/mongo/db/repl/repl_set_config.h
+++ b/src/mongo/db/repl/repl_set_config.h
@@ -552,6 +552,12 @@ public:
      */
     ReplSetConfigPtr getRecipientConfig() const;
 
+    /**
+     * Compares the write concern modes with another config and returns 'true' if they are
+     * identical.
+     */
+    bool areWriteConcernModesTheSame(ReplSetConfig* otherConfig) const;
+
 private:
     /**
      * Sets replica set ID to 'defaultReplicaSetId' if 'cfg' does not contain an ID.
diff --git a/src/mongo/db/repl/repl_set_config_test.cpp b/src/mongo/db/repl/repl_set_config_test.cpp
index be6753afba4..e43bd0385c8 100644
--- a/src/mongo/db/repl/repl_set_config_test.cpp
+++ b/src/mongo/db/repl/repl_set_config_test.cpp
@@ -1931,6 +1931,193 @@ TEST(ReplSetConfig, MakeCustomWriteMode) {
     ASSERT_TRUE(swPattern.isOK());
 }
 
+TEST(ReplSetConfig, SameWriteConcernModesNoCustom) {
+    auto config = ReplSetConfig::parse(BSON("_id"
+                                            << "rs0"
+                                            << "version" << 1 << "term" << 1.0 << "protocolVersion"
+                                            << 1 << "members"
+                                            << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                                                     << "localhost:12345"))));
+
+    auto otherConfig = ReplSetConfig::parse(BSON("_id"
+                                                 << "rs0"
+                                                 << "version" << 2 << "term" << 1.0
+                                                 << "protocolVersion" << 1 << "members"
+                                                 << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                                                          << "localhost:6789"))));
+
+    ASSERT(config.areWriteConcernModesTheSame(&otherConfig));
+    ASSERT(otherConfig.areWriteConcernModesTheSame(&config));
+}
+
+TEST(ReplSetConfig, SameWriteConcernModesOneCustom) {
+    auto config = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY")))
+             << "settings" << BSON("getLastErrorModes" << BSON("eastCoast" << BSON("NYC" << 1)))));
+
+    auto otherConfig = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 2 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:6789"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY")))
+             << "settings" << BSON("getLastErrorModes" << BSON("eastCoast" << BSON("NYC" << 1)))));
+
+    ASSERT(config.areWriteConcernModesTheSame(&otherConfig));
+    ASSERT(otherConfig.areWriteConcernModesTheSame(&config));
+}
+
+TEST(ReplSetConfig, DifferentWriteConcernModesOneCustomDifferentName) {
+    auto config = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY")))
+             << "settings" << BSON("getLastErrorModes" << BSON("somename" << BSON("NYC" << 1)))));
+
+    auto otherConfig = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 2 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:6789"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY")))
+             << "settings" << BSON("getLastErrorModes" << BSON("othername" << BSON("NYC" << 1)))));
+
+    ASSERT_FALSE(config.areWriteConcernModesTheSame(&otherConfig));
+    ASSERT_FALSE(otherConfig.areWriteConcernModesTheSame(&config));
+}
+
+TEST(ReplSetConfig, DifferentWriteConcernModesOneCustomDifferentCounts) {
+    auto config = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY"))
+                           << BSON("_id" << 1 << "host"
+                                         << "otherhost:12345"
+                                         << "tags"
+                                         << BSON("NYC"
+                                                 << "NY")))
+             << "settings" << BSON("getLastErrorModes" << BSON("eastCoast" << BSON("NYC" << 1)))));
+
+    auto otherConfig = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY"))
+                           << BSON("_id" << 1 << "host"
+                                         << "otherhost:12345"
+                                         << "tags"
+                                         << BSON("NYC"
+                                                 << "NY")))
+             << "settings" << BSON("getLastErrorModes" << BSON("eastCoast" << BSON("NYC" << 2)))));
+
+    ASSERT_FALSE(config.areWriteConcernModesTheSame(&otherConfig));
+    ASSERT_FALSE(otherConfig.areWriteConcernModesTheSame(&config));
+}
+
+TEST(ReplSetConfig, DifferentWriteConcernModesExtraTag) {
+    auto config = ReplSetConfig::parse(BSON("_id"
+                                            << "rs0"
+                                            << "version" << 1 << "term" << 1.0 << "protocolVersion"
+                                            << 1 << "members"
+                                            << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                                                     << "localhost:12345"
+                                                                     << "tags"
+                                                                     << BSON("NYC"
+                                                                             << "NY"))
+                                                          << BSON("_id" << 1 << "host"
+                                                                        << "otherhost:12345"
+                                                                        << "tags"
+                                                                        << BSON("Boston"
+                                                                                << "MA")))
+                                            << "settings"
+                                            << BSON("getLastErrorModes"
+                                                    << BSON("nyonly" << BSON("NYC" << 1) << "maonly"
+                                                                     << BSON("Boston" << 1)))));
+
+    auto otherConfig = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY"))
+                           << BSON("_id" << 1 << "host"
+                                         << "otherhost:12345"
+                                         << "tags"
+                                         << BSON("Boston"
+                                                 << "MA")))
+             << "settings" << BSON("getLastErrorModes" << BSON("nyonly" << BSON("NYC" << 1)))));
+
+    ASSERT_FALSE(config.areWriteConcernModesTheSame(&otherConfig));
+    ASSERT_FALSE(otherConfig.areWriteConcernModesTheSame(&config));
+}
+
+TEST(ReplSetConfig, DifferentWriteConcernModesSameNameDifferentDefinition) {
+    auto config = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345"
+                                      << "tags"
+                                      << BSON("NYC"
+                                              << "NY"))
+                           << BSON("_id" << 1 << "host"
+                                         << "otherhost:12345"
+                                         << "tags"
+                                         << BSON("Boston"
+                                                 << "MA")))
+             << "settings" << BSON("getLastErrorModes" << BSON("eastCoast" << BSON("NYC" << 1)))));
+
+    auto otherConfig = ReplSetConfig::parse(BSON(
+        "_id"
+        << "rs0"
+        << "version" << 1 << "term" << 1.0 << "protocolVersion" << 1 << "members"
+        << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                 << "localhost:12345"
+                                 << "tags"
+                                 << BSON("NYC"
+                                         << "NY"))
+                      << BSON("_id" << 1 << "host"
+                                    << "otherhost:12345"
+                                    << "tags"
+                                    << BSON("Boston"
+                                            << "MA")))
+        << "settings" << BSON("getLastErrorModes" << BSON("eastCoast" << BSON("Boston" << 1)))));
+
+    ASSERT_FALSE(config.areWriteConcernModesTheSame(&otherConfig));
+    ASSERT_FALSE(otherConfig.areWriteConcernModesTheSame(&config));
+}
+
 }  // namespace
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/repl_set_tag.h b/src/mongo/db/repl/repl_set_tag.h
index 87dd68bd2ac..420ace32052 100644
--- a/src/mongo/db/repl/repl_set_tag.h
+++ b/src/mongo/db/repl/repl_set_tag.h
@@ -154,6 +154,37 @@ public:
         return _constraints.end();
     }
 
+    /**
+     * Gets the number of constraints in this pattern.
+     */
+    size_t getNumConstraints() const {
+        return _constraints.size();
+    }
+
+    bool operator==(const ReplSetTagPattern& other) const {
+        if (getNumConstraints() != other.getNumConstraints()) {
+            return false;
+        }
+
+        for (auto itrA = constraintsBegin(); itrA != constraintsEnd(); itrA++) {
+            bool same = false;
+            for (auto itrB = other.constraintsBegin(); itrB != other.constraintsEnd(); itrB++) {
+                if (*itrA == *itrB) {
+                    same = true;
+                    break;
+                }
+            }
+            if (!same) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    bool operator!=(const ReplSetTagPattern& other) const {
+        return !operator==(other);
+    }
+
 private:
     std::vector<TagCountConstraint> _constraints;
 };
diff --git a/src/mongo/db/repl/replication_coordinator.h b/src/mongo/db/repl/replication_coordinator.h
index 8cb3d976242..01966a7722a 100644
--- a/src/mongo/db/repl/replication_coordinator.h
+++ b/src/mongo/db/repl/replication_coordinator.h
@@ -1147,6 +1147,28 @@ public:
      */
     virtual void recordIfCWWCIsSetOnConfigServerOnStartup(OperationContext* opCtx) = 0;
 
+    /**
+     * Interface used to synchronize changes to custom write concern tags in the config and
+     * custom default write concern settings.
+     *
+     * Use [reserve|release]DefaultWriteConcernChanges when making changes to the current
+     * default read/write concern.
+     * Use [reserve|release]ConfigWriteConcernTagChanges when executing a reconfig that
+     * could potentially change read/write concern tags.
+     */
+    class WriteConcernTagChanges {
+    public:
+        WriteConcernTagChanges() = default;
+        virtual ~WriteConcernTagChanges() = default;
+        virtual bool reserveDefaultWriteConcernChange() = 0;
+        virtual void releaseDefaultWriteConcernChange() = 0;
+
+        virtual bool reserveConfigWriteConcernTagChange() = 0;
+        virtual void releaseConfigWriteConcernTagChange() = 0;
+    };
+
+    virtual WriteConcernTagChanges* getWriteConcernTagChanges() = 0;
+
 protected:
     ReplicationCoordinator();
 };
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index 0805cb25fe1..23cae7e6c1a 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -149,6 +149,8 @@ MONGO_FAIL_POINT_DEFINE(hangAfterReconfig);
 MONGO_FAIL_POINT_DEFINE(skipBeforeFetchingConfig);
 // Hang after grabbing the RSTL but before we start rejecting writes.
 MONGO_FAIL_POINT_DEFINE(stepdownHangAfterGrabbingRSTL);
+// Hang before making checks on the new config relative to the current one.
+MONGO_FAIL_POINT_DEFINE(hangBeforeNewConfigValidationChecks);
 // Simulates returning a specified error in the hello response.
 MONGO_FAIL_POINT_DEFINE(setCustomErrorInHelloResponseMongoD);
 // Throws right before the call into recoverTenantMigrationAccessBlockers.
@@ -3690,6 +3692,27 @@ Status ReplicationCoordinatorImpl::_doReplSetReconfig(OperationContext* opCtx,
         return status;
     ReplSetConfig newConfig = newConfigStatus.getValue();
 
+    // Synchronize this change with potential changes to the default write concern.
+    auto wcChanges = getWriteConcernTagChanges();
+    auto mustReleaseWCChange = false;
+    if (!oldConfig.areWriteConcernModesTheSame(&newConfig)) {
+        if (!wcChanges->reserveConfigWriteConcernTagChange()) {
+            return Status(
+                ErrorCodes::ConflictingOperationInProgress,
+                "Default write concern change(s) in progress. Please retry the reconfig later.");
+        }
+        // Reservation OK.
+        mustReleaseWCChange = true;
+    }
+
+    ON_BLOCK_EXIT([&] {
+        if (mustReleaseWCChange) {
+            wcChanges->releaseConfigWriteConcernTagChange();
+        }
+    });
+
+    hangBeforeNewConfigValidationChecks.pauseWhileSet();
+
     // Excluding reconfigs that bump the config term during step-up from checking against changing
     // the implicit default write concern, as it is not needed.
     if (!skipSafetyChecks /* skipping step-up reconfig */) {
@@ -3740,6 +3763,30 @@ Status ReplicationCoordinatorImpl::_doReplSetReconfig(OperationContext* opCtx,
                 }
             }
         }
+
+        // If we are currently using a custom write concern as the default, check that the
+        // corresponding definition still exists in the new config.
+        if (serverGlobalParams.clusterRole == ClusterRole::None) {
+            try {
+                const auto rwcDefaults =
+                    ReadWriteConcernDefaults::get(opCtx->getServiceContext()).getDefault(opCtx);
+                const auto wcDefault = rwcDefaults.getDefaultWriteConcern();
+                if (wcDefault) {
+                    auto validateWCStatus = newConfig.validateWriteConcern(wcDefault.get());
+                    if (!validateWCStatus.isOK()) {
+                        return Status(ErrorCodes::NewReplicaSetConfigurationIncompatible,
+                                      str::stream() << "May not remove custom write concern "
+                                                       "definition from config while it is in "
+                                                       "use as the default write concern. "
+                                                       "Change the default write concern to a "
+                                                       "non-conflicting setting and try the "
+                                                       "reconfig again.");
+                    }
+                }
+            } catch (const DBException& e) {
+                return e.toStatus("Exception while loading write concern default during reconfig");
+            }
+        }
     }
 
     BSONObj oldConfigObj = oldConfig.toBSON();
@@ -6159,5 +6206,10 @@ void ReplicationCoordinatorImpl::_validateDefaultWriteConcernOnShardStartup(With
     }
 }
 
+ReplicationCoordinatorImpl::WriteConcernTagChanges*
+ReplicationCoordinatorImpl::getWriteConcernTagChanges() {
+    return &_writeConcernTagChanges;
+}
+
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/replication_coordinator_impl.h b/src/mongo/db/repl/replication_coordinator_impl.h
index 8affe603f30..4d24f253799 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.h
+++ b/src/mongo/db/repl/replication_coordinator_impl.h
@@ -518,6 +518,74 @@ public:
      */
     void cancelElection_forTest();
 
+    /**
+     * Implementation of an interface used to synchronize changes to custom write concern tags in
+     * the config and custom default write concern settings.
+     * See base class fore more information.
+     */
+    class WriteConcernTagChangesImpl : public WriteConcernTagChanges {
+    public:
+        WriteConcernTagChangesImpl() = default;
+        virtual ~WriteConcernTagChangesImpl() = default;
+
+        bool reserveDefaultWriteConcernChange() override {
+            stdx::lock_guard lock(_mutex);
+            if (_configWriteConcernTagChanges > 0) {
+                return false;
+            }
+            _defaultWriteConcernChanges++;
+            return true;
+        }
+
+        void releaseDefaultWriteConcernChange() override {
+            stdx::lock_guard lock(_mutex);
+            invariant(_defaultWriteConcernChanges > 0);
+            _defaultWriteConcernChanges--;
+        }
+
+        bool reserveConfigWriteConcernTagChange() override {
+            stdx::lock_guard lock(_mutex);
+            if (_defaultWriteConcernChanges > 0) {
+                return false;
+            }
+            _configWriteConcernTagChanges++;
+            return true;
+        }
+
+        void releaseConfigWriteConcernTagChange() override {
+            stdx::lock_guard lock(_mutex);
+            invariant(_configWriteConcernTagChanges > 0);
+            _configWriteConcernTagChanges--;
+        }
+
+    private:
+        //
+        // All member variables are labeled with one of the following codes indicating the
+        // synchronization rules for accessing them.
+        //
+        // (R)  Read-only in concurrent operation; no synchronization required.
+        // (S)  Self-synchronizing; access in any way from any context.
+        // (PS) Pointer is read-only in concurrent operation, item pointed to is self-synchronizing;
+        //      Access in any context.
+        // (M)  Reads and writes guarded by _mutex
+        // (I)  Independently synchronized, see member variable comment.
+
+        // The number of config write concern tag changes currently underway.
+        size_t _configWriteConcernTagChanges{0};  // (M)
+
+        // The number of default write concern changes currently underway.
+        size_t _defaultWriteConcernChanges{0};  // (M)
+
+        // Used to synchronize access to the above variables.
+        Mutex _mutex = MONGO_MAKE_LATCH(
+            "ReplicationCoordinatorImpl::PendingWriteConcernTagChangesImpl::_mutex");  // (S)
+    };
+
+    /**
+     * Returns a pointer to the WriteConcernTagChanges used by this instance.
+     */
+    WriteConcernTagChanges* getWriteConcernTagChanges() override;
+
 private:
     using CallbackFn = executor::TaskExecutor::CallbackFn;
 
@@ -1766,6 +1834,10 @@ private:
     boost::optional<bool> _wasCWWCSetOnConfigServerOnStartup;
 
     InitialSyncerInterface::OnCompletionFn _onCompletion;
+
+    // Construct used to synchronize default write concern changes with config write concern
+    // changes.
+    WriteConcernTagChangesImpl _writeConcernTagChanges;
 };
 
 }  // namespace repl
diff --git a/src/mongo/db/repl/replication_coordinator_mock.cpp b/src/mongo/db/repl/replication_coordinator_mock.cpp
index 0dfcd6e66e1..d396c2c429d 100644
--- a/src/mongo/db/repl/replication_coordinator_mock.cpp
+++ b/src/mongo/db/repl/replication_coordinator_mock.cpp
@@ -782,5 +782,11 @@ void ReplicationCoordinatorMock::recordIfCWWCIsSetOnConfigServerOnStartup(Operat
     MONGO_UNREACHABLE;
 }
 
+ReplicationCoordinatorMock::WriteConcernTagChanges*
+ReplicationCoordinatorMock::getWriteConcernTagChanges() {
+    MONGO_UNREACHABLE;
+    return nullptr;
+}
+
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/replication_coordinator_mock.h b/src/mongo/db/repl/replication_coordinator_mock.h
index a03227ce33e..a77d5ad4982 100644
--- a/src/mongo/db/repl/replication_coordinator_mock.h
+++ b/src/mongo/db/repl/replication_coordinator_mock.h
@@ -404,6 +404,21 @@ public:
 
     virtual void recordIfCWWCIsSetOnConfigServerOnStartup(OperationContext* opCtx) final;
 
+    class WriteConcernTagChangesMock : public WriteConcernTagChanges {
+        virtual ~WriteConcernTagChangesMock() = default;
+        virtual bool reserveDefaultWriteConcernChange() {
+            return false;
+        };
+        virtual void releaseDefaultWriteConcernChange() {}
+
+        virtual bool reserveConfigWriteConcernTagChange() {
+            return false;
+        };
+        virtual void releaseConfigWriteConcernTagChange() {}
+    };
+
+    virtual WriteConcernTagChanges* getWriteConcernTagChanges() override;
+
 private:
     ServiceContext* const _service;
     ReplSettings _settings;
diff --git a/src/mongo/db/repl/replication_coordinator_noop.cpp b/src/mongo/db/repl/replication_coordinator_noop.cpp
index 9ca8ffe1d9d..66f46c4fd92 100644
--- a/src/mongo/db/repl/replication_coordinator_noop.cpp
+++ b/src/mongo/db/repl/replication_coordinator_noop.cpp
@@ -607,5 +607,11 @@ void ReplicationCoordinatorNoOp::recordIfCWWCIsSetOnConfigServerOnStartup(Operat
     MONGO_UNREACHABLE;
 }
 
+ReplicationCoordinatorNoOp::WriteConcernTagChanges*
+ReplicationCoordinatorNoOp::getWriteConcernTagChanges() {
+    MONGO_UNREACHABLE;
+    return nullptr;
+}
+
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/replication_coordinator_noop.h b/src/mongo/db/repl/replication_coordinator_noop.h
index 635ac26d371..baf4b586d2b 100644
--- a/src/mongo/db/repl/replication_coordinator_noop.h
+++ b/src/mongo/db/repl/replication_coordinator_noop.h
@@ -332,6 +332,21 @@ public:
 
     virtual void recordIfCWWCIsSetOnConfigServerOnStartup(OperationContext* opCtx) final;
 
+    class WriteConcernTagChangesNoOp : public WriteConcernTagChanges {
+        virtual ~WriteConcernTagChangesNoOp() = default;
+        virtual bool reserveDefaultWriteConcernChange() {
+            return false;
+        };
+        virtual void releaseDefaultWriteConcernChange() {}
+
+        virtual bool reserveConfigWriteConcernTagChange() {
+            return false;
+        };
+        virtual void releaseConfigWriteConcernTagChange() {}
+    };
+
+    virtual WriteConcernTagChanges* getWriteConcernTagChanges() override;
+
 private:
     ServiceContext* const _service;
 };
diff --git a/src/mongo/embedded/replication_coordinator_embedded.cpp b/src/mongo/embedded/replication_coordinator_embedded.cpp
index 53e95a0739c..d0182bfce59 100644
--- a/src/mongo/embedded/replication_coordinator_embedded.cpp
+++ b/src/mongo/embedded/replication_coordinator_embedded.cpp
@@ -636,5 +636,10 @@ void ReplicationCoordinatorEmbedded::recordIfCWWCIsSetOnConfigServerOnStartup(
     MONGO_UNREACHABLE;
 }
 
+ReplicationCoordinatorEmbedded::WriteConcernTagChanges*
+ReplicationCoordinatorEmbedded::getWriteConcernTagChanges() {
+    UASSERT_NOT_IMPLEMENTED;
+}
+
 }  // namespace embedded
 }  // namespace mongo
diff --git a/src/mongo/embedded/replication_coordinator_embedded.h b/src/mongo/embedded/replication_coordinator_embedded.h
index b6c5bfe1c62..f2d29ddea37 100644
--- a/src/mongo/embedded/replication_coordinator_embedded.h
+++ b/src/mongo/embedded/replication_coordinator_embedded.h
@@ -342,6 +342,21 @@ public:
 
     virtual void recordIfCWWCIsSetOnConfigServerOnStartup(OperationContext* opCtx) final;
 
+    class WriteConcernTagChangesEmbedded : public WriteConcernTagChanges {
+        virtual ~WriteConcernTagChangesEmbedded() = default;
+        virtual bool reserveDefaultWriteConcernChange() {
+            return false;
+        };
+        virtual void releaseDefaultWriteConcernChange() {}
+
+        virtual bool reserveConfigWriteConcernTagChange() {
+            return false;
+        };
+        virtual void releaseConfigWriteConcernTagChange() {}
+    };
+
+    virtual WriteConcernTagChanges* getWriteConcernTagChanges() override;
+
 private:
     // Back pointer to the ServiceContext that has started the instance.
     ServiceContext* const _service;